refactor(frontend): satisfy noUncheckedIndexedAccess (S3b)

Enable noUncheckedIndexedAccess and fix all 98 call sites it surfaced across 18
files. Every fix is behaviour-preserving — the flag flagged reads TypeScript
couldn't prove in-bounds, all of which were already guarded by a length/index check,
a findIndex result, or a non-empty invariant:

- read-once so a ternary's narrowing sticks (Feed overrides, linkify mention);
- non-null assertion AFTER an existing guard (PlayerModal/VideoEditor queue+cut-list
  indexing, modules step, useUndoable stacks) with a comment stating the invariant;
- nullish fallback where undefined is a real possibility (GlassTuner slider ?? def,
  ConfigPanel active group ?? [], descriptionLinks id ?? null);
- optional chaining where the entry genuinely can be absent (PlexBrowse IO entry);
- a non-empty tuple type for the RELATIVE_UNITS constant (encodes "always has [0]").

The single highest-leverage fix: PlayerModal's `active` became `queue?.[index] ??
video` (always Video), clearing 34 of the 45 errors in that file at once. Verified:
tsc/eslint/prettier clean, 56 vitest + 34 pytest + 17/17 e2e green (the e2e suite
exercises the feed/player/channel components touched here).

frontend/src/components/Feed.tsx

@@ -330,8 +330,14 @@ export default function Feed({
   loadedRef.current = loaded;
   const withOverrides = (list: Video[]): Video[] =>
     list
-      .map((v) => (overrides[v.id] ? { ...v, status: overrides[v.id] } : v))
-      .map((v) => (savedOverrides[v.id] !== undefined ? { ...v, saved: savedOverrides[v.id] } : v));
+      .map((v) => {
+        const status = overrides[v.id]; // read once so the narrowing sticks
+        return status ? { ...v, status } : v;
+      })
+      .map((v) => {
+        const saved = savedOverrides[v.id];
+        return saved !== undefined ? { ...v, saved } : v;
+      });
   const merged: Video[] = withOverrides(loaded);
   const items: Video[] = merged.filter((v) => matchesView(v.status, filters.show));
   // The in-app player's queue (auto-advance / loop / prev-next). It's the filtered feed, but the